1. Field of the Invention
This invention relates to solar thermal power generation, and in particular, to an improved focusing solar collector system for cogenerating heat and electricity.
2. Description of the Prior Art
One of the major limiting factors associated with the large scale generation of power using solar energy is the fact that the incident energy density of solar radiation is low, so that solar energy power plants must collect light over a large surface area. Thus, the capital expense for the collecting apparatus and power conversion device is the dominant consideration in determining the ultimate cost per kilowatt hour of electricity, since the sunlight is free. One important effort for developing solar power includes the conversion of thermal energy into electricity by a large number of distributed collectors which employ a moderate degree of concentration. In this approach, the incident solar radiation is enhanced roughly by a factor of fifty to five hundred when focusing reflectors are used, making it possible to obtain operating temperatures over the range of 500.degree. C. to 1,000.degree. C.
A working fluid heated to temperatures in this operating range can be used to produce electric power, for example by generating steam to power a steam turbine electric generator. The concentration of the incident solar radiation to obtain such high operating temperatures is provided by focusing collectors which include a concentrator, a receiver and an absorber, which is usually part of the receiver.
The intense focal energy influx in focusing collectors causes high receiver temperatures and intense thermal radiation. One approach to reducing these losses is to provide a receiver coating material having high absorptivity to solar radiation and low infrared emissivity at the receiver operating temperature. Efficiency has also been increased by choosing a collector geometry which makes the absorbing (irradiated) area large and the emitting area small. By choosing the right combination of receiver surface materials, receiver geometry and heat transfer efficiency, it is possible to operate at high concentration ratios, and extract a high fraction of the incident solar radiation focused on the system by the concentrator.
Even assuming that the distributed collectors are operated at high efficiency, the overall power generation system is limited significantly by the fact that it can operate only during daylight hours. Because of the limited collection period, the power generated can only be used intermittently to augment conventional power generating systems, or some means of storage of the energy must be provided. Additionally, some provision must be made for disposing of the waste heat associated with the power generating operation.